Degaussing Coil To Restore Gameplay Like It’s 1985

You may think that cathode ray tube (CRT) TVs and monitors have gone the way of the dinosaur, but you’d be wrong. Many still have them for playing video games at home or in arcades, for vintage computing, and yes, even for watching television programs. [Nesmaniac] uses his TV for playing Super Mario Bros but for several years it had a red area in the top right corner due to a nearby lightning strike. Sadly, it stood out particularly well against the game’s blue background. His solution was to make a degaussing coil.

We have an article explaining degaussing in detail but in brief, the red was caused by that area of the metal shadow mask at the front of the display becoming magnetized by the lightning strike. One way to get rid of the red area is to bring a coil near it and gradually move the coil away. The coil has AC from a wall socket running through it, producing an oscillating magnetic field which randomizes the magnetic field on the shadow mask, restoring the colors to their former glory.

You’ll find [Nesmaniac’s] video explaining how he made it below. It’s a little cartoonish but the details are all there, along with the necessary safety warnings. His degaussing coil definitely qualifies as a hack. The coil itself came from a 15″ CRT monitor and his on/off switch came from a jigsaw. A 100 watt light bulb serves as a resistance to minimize current and if more or less current is needed then the bulb can be swapped for one with a different wattage.

To demonstrate it in action and give a few more construction details, we’ve included a second video below by [Arcade Jason] who made his for degaussing arcade game screens.

Our thanks to [example] for the tip about this and reminding us that degaussing a CRT is still sometimes necessary.

Because LCDs fail at being CRTs. For example, they tend to have internal refresh rates and some kind of frame rate converter (for example throw one out of 6 if it’s 60 to 50, or duplicate one out of 5 if it’s 50 to 60). When frame rates don’t match perfectly, this creates annoying jitter in smooth moving backgrounds and sprites.

CRTs directly refresh the screen from the input signal while LCDs have a frame buffer and some sort of image processing pipeline. In general LCDs add noticeable amounts of latency, often even in “Game” mode with the post-processing turned off.

In real operation the CRT degaussing coil is used without an additional resistor (except the PTC which disables it gradually to do it’s work). When I worked in a TV repair shop about 1988 or 1990 we had also a handheld degaussing coil that was plugged straight in to the 220V mains and had a simple switch. But I think it was only rated for short time operation.

Yes, there is an internal degaussing coil powered via a series PTC. But these PTCs age and fail over time. Basically, there are three scenarios in which the internal PTC can fail:

1) It can short-circuit. The degaussing coil will then draw an exessive amount of current, which blows the internal fuse of the TV set – sometimes even the mains fuse in the electricity panel of your house/apartment/flat.

2) It can stay at very high resistance when cold. The TV will work just fine, except that there is no degaussing taking place any more.

3) Somewhere in between (1) and (2): The PTC might keep a relatively high resistance when cold, yet enough to still provide a little bit of “degaussing”. In this case the PTC is likely to fail with either of the first two scenarios very soon. If it is in that stage, degaussing might not fully work anymore due to non-linearities in the about-to-fail PTC, resulting not in a de-magnetized but a “pre-“magnetized hole mask which leads to a blotchy picture of mostly false colours.

Problem (3) looks quite similar to a failing CRT tube or problems in the color decoder IC. If a normal picture reappears while holding a magnet in front of the screen, the PTC can be suspected to have failed.

In short: Yes, I suspect that replacing the PTC would resolve the issue and revive the TV set. One could also go and replace some of the old capacitors of the power supply and around the flyback transformer. However, not everybody feels happy about working on the internals of a TV set, considering all these (probably) charged capacitors and (potentially lethal) high voltages inside.

I have worked in the internals of TV sets at a time when CRTs where the way to view pictures. But to day I would not invest any time or parts in that outdated technology. In the meantime here the analog TV stations are gone.

Most of the colour TV’s had this. So you could temporarily mess the colour with some high strength magnets leaving some residual field on the (iron/steel) shadow mask. Some people freaked as it seemed to stay there after another (quick) power cycle but, that was just an indication the auto degaussing circuit feeble Or power saving. Repeat the power up a dozen times over 2-3 days and it was fine…
Fwiw. The good thing about the degaussing coil with suitable PTC was the declining field oscillation well matched to properties of the shadow mask permeabilty. Iirc most shadow masks were a better grade steel something like low level stainless but, some had real cheap iron which meant it accepted the neodymium type magnets impression far too easily and in some cases might have physically warped the shadow mask. I never saw evidence of this. All those I played with recovered after at most 3 days power cyclicing though did need long enough for the PTC to cool down well.

I still have one laying around, salvaged from an old VGA monitor.
It has the usual PTC in series. In the monitor, it was switched on with the mains switch, and powered all the time.
I used it to degauss my CRT TV, when I still had one.

Ahh degaussing. This was one of my favorite tricks when people would complain that their TVs were getting crazy colors in one corner. I wound a coil around a loop of plastic attached to a screwdriver handle. It was like magic to everyone who watched it.

I also used to love messing with the other kids in the computer lab in school. Our CRT monitors had automatic degaussing, but you could also activate it manually through the on-screen menu. You could really freak a kid out if you timed it just right. One time the kid across the row from me kept kicking me under the table. I brought up the menu and hit the button just as I started yelling at him. He thought I had some kind of supernatural power that my voice distorted his monitor. In reality, the degaussing coil in the monitors was powerful enough to disrupt the display on many adjacent monitors as well.

Most TV’s of the day had a “built in” coil wrapped around the front edge of the CRT. When you powered on the tv, it was connected to the ac line, via a thermal circuit that would slowly open when it warmed up. A lot of times, that device would melt and have to be replaced. Most built in coils really weren’t high enough to fix a “blob” of color. I know in the 70’s it wasn’t uncommon to get a service call for the color is mess up, and you’d find a mega speaker on top or next to the TV. You’d ask them what did they play? Something LOUD with a LOT of bass. I use to carry an old speaker in my service vehicle, along with a high power degaussing coil. I’d show them why the speakers needed to be moved AWAY from the tv by taking the speaker and placing it close to the tv. Then I’d degause the picture tube and typically I’d never get another call for that problem.

The coil built into CRTs generally was not powerful enough to fix the “damage” done by a strong magnet brought near the screen. It was there primarily to deal with magnetism that built up from earth’s own field. This would usually present itself if you had relocated the TV and the screen is now facing a different orientation in relation to earth’s own magnetic field than it did previously. I am not sure that i ever saw a color CRT that did not have a coil, but BW TVs never did because they did not have a shadow mask that would be affected.

I have used a NdFeBo magnet rotating on the end of a power screwdriver to deguass old monitors. Approach slowly, move around in an outward spiral and retreat slowly. Old harddrive magnets mounted(stuck on) by the long side and on end vertically worked best. YMMV

This is how I remember doing it. A few magnets from a dead HDD, a paint stirrer, a bolt and nut, a power drill and some good tape. Fairly common to have around the house for anyone that’s done tech stuff for any duration of time.

Many folks do not know you are referring to Neodymium magnets.
NDFeBo….Neodymium Iron Boron.

I collect as many as I can, for projects.
Gone, are the usefulness of the Alnico types. I call these, the NiCD batteries of the magnet world.
Too weak by design, but you can boost them by pairing them with Neodymium magnets.

No need for any lights in a degaussing coil..add more wire.
Transformer logic…2 Volts per turn…add a few more for good measure.

Find ab “E+I” transformer, open the magnetic core – renove the “I” cores. Connect its primary in series with a filament-based light bulb and connect the resulting circuit to main supply – 220V or 110V. Voila – a degauss device. Short-circuit the secondary coil for greater effect

Well as most have already said the majority of tubes had a degaussing coil around the front of the tube.
Also you could just turn the monitor/TV on and off again to give them more of a kick to get rid of the stubborn stains ;-) !

Also the workshop degauss coil often had a bulb to operate as the PTC element it’s a matter of choice if you are building it your self ..a nice anti prat method if you like. The same method of using bulbs as current limiting resistors was also a big thing in general for testing unknown faulty TVs and further back valve based kit.